Method of manufacturing ferromagnetic bodies and products thereof



Aug. 30, 1960 c. J. ESVELDT ETAL 2,951,042

METHOD OF MANUFACTURING FERROMAGNETIC BODIES AND PRODUCTS THEREOF 5Sheets-Sheet 1 Filed March 27, 1956 max H FIG.1

Aug. 30, 1960 c. J. ESVELDT ETAL 2,951,042

METHOD OF MANUFACTURING FERROMAGNETIC BODIES AND PRODUCTS THEREOF FiledMarch 27, 1956 5 Sheets-Sheet 2 A U 8 III -8 3 a a I o -8 I o (\l L5 U.

INVENTOR CORNELIS JACOBUS ESVELDT HENDRIK' VAN DER HEIDE HENRICUS PETRUSJOHANNES AGENT Aug. 30, 1960 C. J. ESVELDT ET AL METHOD OF MANUFACTURINGFERROMAGNETIC BODIES AND PRODUCTS THEREOF File d March 27, 1956 H(ocrstcd) 5 Sheets-Sheet 3 B-H (gauss) INVENTOR CORNEUS JACOBUS ESVELDTHENDRIK VAN DER HEIDE HENRICUS PETRUS JOHANNES AGENT 1960 c. J. E'SVELDTETAL METHOD OF MAN 2,951,042 UFACTURING FERROMAGNETIC BODIES ANDPRODUCTS THEREOF 5 Sheets-Sheet 4 Filed March 27, 1956 mLUO Av: 00. m

United States l\/IETHOD OF MANUFACTURING FERROMAG- NETIC BODIES ANDPRODUCTS THEREOF Filed Mar. 27, 1956, Ser. No. 574,150

Claims priority, application Netherlands Mar. 28, 1955 7 Claims. (Cl.252-625) Our invention relates to a method of manufacturing aferromagnetic body comprising cobalt ferrous ferrite.

More particularly our invention relates to a method for manufacturingmagnet cores suitable for use in the devices described by W. N. Papian(Proceedings of the Institute of Radio-Engineers, April 1952, pages475-478) and by D. R. Brown and E. Albers Shoenberg (Electronics, April1953, pages 147-149). The magnet cores used for these devices arerequired to have a squareness ratio, (R whose value approaches 1 as nearas possible.

Fig. 1 is a magnetization curve showing the relationship of certainvalues referred to hereinafter. Figs. 2 to 5 are magnetization curves ofmaterials made in accordance with the invention.

For the meaning of the magnitude (RQ reference is made to the literaturementioned above. For completeness sake a brief explanation is givenhereinafter with reference to Fig. 1, a diagrammatical representation ofpart of a magnetizing curve relating to a case in which thedemagnetization started before the magnetic saturation had been reached.The magnitude (R is de fined by:

(1/2Hm) B m)max The quotient is a function of the maximum field-strengthH applied during magnetization. It is found that this quotient has amaximum value at a particular value of H which, as a rule, differslittle from the coercivity H This maximum value of the quotient isindicated by the symbol (R h The measurements of Buzz) and B 1/2Hm)required to determine (R were made, when the invention was done, onannular magnet cores having a constant section of ferromagnetic materialthroughout the periphery of the ring and an outer diameter of not morethan 1.6 times the inner diameter.

With devices of the aforesaid kind not only the squareness ratio (R isimportant, but also a high value of the ratio between the flanksteepnesses of the branches I and II of the hysteresis loop. This ratiois defined by the quotient:

tan a tan ,8 (see also Fig. 1).

Not only for the aforesaid devices, but also, for example, for magneticdrums, for recording coded information and, furthermore, for chokes tolimit power, the inductance of which increases very strongly andabruptly, when the current passing the coil exceeds a prescribed value,use may be made of the bodies manufactured in accordance with theinvention. The bodies produced in atent accordance with the inventionmay, moreover, have an asymmetrical hysteresis loop, so that thecoercivity measured in one direction diflfers from that measured in theother direction, which may be of'importance for certain uses. It isimportant also for these applications that the coercivity should notassume an excessively high value and should remain at least below 100Oersted.

As these rectangular hysteresis loop ferromagnetic materials are almostalways used with alternating currents of high frequencies it isimportant to minimize the occurrence of eddy currents. In the pastattempts have been made to solve this problem by building up magnetcores from insulated, very thin layers of ferromagnetic materials. Forhigh frequencies it was necessary to use as the raw material for themagnet cores a magnetically weak, ferric oxide material having a spinelstructure.

However, even with the use of these magnetically weak net core having ahigh value of the aforementioned ratio tan a tan ,8

It is a further object of our invention to provide a magnet core havingan asymmetrical hysteresis loop in which the coercivity measured in onedirection differs from the coercivity measured in the other direction.

According to our invention a ferromagnetic body is produced from cobaltferrous ferrite in which and H 100 Oersted. More particularly our methodinvolves the steps of sintering a mixture, moulded in the desired form,of at least one cobalt pound and at least one iron compound (oxidesand/or compounds capable of producing oxides by heating) in acomposition according to an atomic ratio of CozFe between 0.007:1 and0.4:1 in reducing conditions at a temperature exceeding 1200 C., until abody having a pore volume of not more than 10%, i.e., a body whosedensity is at least of theoretical density has been obtained, coolingthis body to about room temperature and then treating the cooled body byheating it to a temperature of more than 250 C., and finally coolingthis body in a director alternating magnetic field. The last-mentionedaftertreatment is described for other ferromagnetic material inElectrical Engineering, May 1951, pages 420421.

The term reducing conditions, referred to above, is to be understood tomean herein conditions, in which part of the iron gets into or remainsin the bivalent condition. Whether we are concerned with reducingconditions or not, depends upon the oxygen dissociation pressure of thereaction mass and upon the partial oxygen pressure of the ambient gasatmosphere. At a sufiicient high sintering temperature (for example of1450 0.) oxygen of a pressure of 1 atmosphere may, in this sense, be areducing condition, since at these high temperatures the oxygendissociation pressure of the reaction mass is, as a rule, more than 1atmosphere.

The invention will be described with reference to a few embodiments.

.3 Example I A mixture of cobalt oxide and iron oxide in a ratio of 0.36gram atom of cobalt on 1 gram atom of iron is moulded to rings, whichare heated for two hours at 1450 C; in air and then cooled to roomtemperature. According to a chemical analysis the product thus obtainedcontains 2.9% by weight of bivalent iron. The (RQ of the sintered bodiesappears to be negative. The rings are then heated in air to 600 C. andcooled in a direct magnetic field of about 100 Oersted. The (R of therings thus treated is between 0.85 and 0.90 and the coercivity H isabout 45 Oersted.

Example II A mixture of cobalt oxide and iron oxide in the ratio of 0.25gram atom of cobalt on 1 gram atom of iron is moulded to obtain ringswhich are heated for two hours at 1450 C. in oxygen under a pressure of1 atmosphere and then cooled. The product thus obtained contains 8.3 byweight of bivalent iron and has a pore volume of 3.6%. At roomtemperature the rings exhibit strongly grooved hysteresis loops at amaximum field strength of less than about 1000 Oersted. Fig. 2 showssuch a hysteresis loop, as well as hysteresis loop for much higher fieldstrengths up to more than 2000 Oersted. The (12,)... of the rings is nowfound to be approximately zero. Then the rings are heated in air toabout 600 C. and then cooled in a direct magnetic field of about 80Oersted. The hysteresis loops of the rings thus treated are shown inFig. 3. The value of (R max has increased to about 0.95. The coercivityH is now about 80 Oersted.

Example III A mixture of cobalt oxide and iron oxide in the ratio of0.15 gram atom of cobalt on 1 gram atom of iron is moulded to obtainrings, which are heated for two hours in a gas stream of 80% by volumeof carbon dioxide, 18% by volume of nitrogen and 2% by volume ofhydrogen to 1350 C., after which they are cooled. The product thusobtained contains 14.0% by weight of bivalent iron and the pore volumeis 3.0%. At room temperature the rings exhibit grooved hysteresis loopsaccording to those shown in Fig. 2. The (R of the rings is in this casenegative. Then the rings are heated in carbon dioxide to about600 C. andthen cooled in an alternating magnetic field of 60 Oersted. The (R ofthe rings is then found to be 0.90 at field strengths between 30 and 60Oersted.

Example IV A mixture of cobalt oxide and iron oxide in a ratio of 0.03gram atom of cobalt on 1 gram atom of iron is moulded to obtain rings,which are heated for two hours at 1350 C. in a gas stream of acomposition of 75% by volume of carbon dioxide, 22.5% by volume ofnitrogen and 2.5% by volume of hydrogen and then cooled. The productthus obtained has a content of 22% by weight of bivalent iron and a porevolume of 1.5%.

For two diiferent amplitudes of the field strength Fig. 4 showshysteresis loops of the rings produced. It is found that (R is aboutzero. The rings are heated in carbon dioxide to 600 C. and then cooledin a direct magnetic field of about 35 Oersted. A hysteresis loop of aring thus after-treated is shown in Fig. 5. The (R then has a value ofabout 0.95 and the coercivity is 9.5

Oersted.

Example V A mixture of cobalt oxide and iron oxide in a ratio of 0.017gram atom of cobalt on 1 gram atom of iron is moulded to obtain rings,which are heated for two hours to 1350 C. in a gas stream of 8.3% byvolume of carbon dioxide, 15.3% by volume of nitrogen and 1.4% by volumeof hydrogen and then cooled. The sintered bodies thus obtained are foundto have an (R of 0.25. The content of bivalent iron is 24% by weight andthe pore volume is 3.6%. One of the rings is then heated in carbondioxide to about 600 C. and then cooled in a direct magnetic field ofabout 50 Oersted. Owing to this treatment the (R of the ring is found tohave increased to 0.99. The coercivity of the ring is 7 Oersted. Afurther ring is heated in carbon dioxide to 600 C. and cooled in analternating magnetic field of 160 Oersted. The ring thus treatedexhibits an (R of 0.95 and a coercivity of 5 Oersted.

While we have described our invention in connection with specificembodiments and applications, other modiiications thereof will bereadily apparent to those skilled in this art without departing from thespirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. A ferromagnetic ferrite body having a substantially square hysteresisloop formed by firing a compacted mixture of cobalt and iron oxides inproportions at which the atomic ratio CozFe is between about 0.007:1 and0.411 at a temperature of about 1350 to 1450 C. under reducingconditions to form a coherent body having a density exceeding oftheoretical density, and cooling said body from a temperature exceeding250 C. in a magnetic field to thereby form a body having an (R of atleast 0.85 and a coercive force of not more than oersted.

2. A method of manufacturing a ferromagnetic ferrite having asubstantially square hysteresis loop comprising the steps, forming acompacted mixture of cobalt and iron oxides in proportions at which theatomic ratio CozFe is between about 0.007z1 and 04:1, heating saidmixture to a temperature of about 1350 to 1450 C. under reducingconditions to form a coherent body having a density not less than 90% oftheoretical density, and cooling said body from a temperature exceeding250 C. in a magnetic field whereby a body is formed having an (Rp of atleast 0.85 and a coercive force not exceeding 100 oersted.

3. A method of manufacturing a ferromagnetic ferrite having asubstantially square hysteresis loop comprising the steps, forming acompacted mixture of cobalt and iron oxides in proportions at which theatomic ratio CozFe is between about 0.007:1 and 0.4: 1, heating saidmixture to a temperature ofabout 1350 to 1450 C. under reducingconditions to form a coherent body having a density not less than 90% oftheoretical density, and cooling said body from a temperature of about600 C. in a magnetic field whereby a body is formed having an (R of atleast 0.85 and a coercive force not'exceeding 100 oersted.

4. A method of manufacturing a ferromagnetic ferrite having asubstantially square hysteresis loop comprising the steps, forming acompacted mixture of cobalt and iron oxides in proportions at which theatomic ratio CozFe is between about 0.007:l and 0.411, heating saidmixture to a temperature of about 1350" to 1450 C. under reducingconditions to form a coherent body having a density not less than 90% oftheoretical density, heating said body from a temperature of about 600C. in an atmosphere of carbon dioxide, and cooling said body in amagnetic field whereby a body is formed having an (R of at least 0.85and a coercive force not exceeding 100 oersted.

5. A method of manufacturing a ferromagnetic ferrite having asubstantially square hysteresis loop comprising the steps, forming acompacted mixture of cobalt and iron oxides in proportions at which theatomic ratio CorFe is between about 0.00721 and 0.411, heating saidmixture to a temperature of about 1350 to 1450 C. in an oxygenatmosphere to form a coherent body having a density not less than 9.0%of theoretical density, heating said body to a temperature of about 600C. in an atmopshere of carbon dioxide, and cooling said body in amagnetic field whereby a body is formed having an (R max of at least0.85 and a coercive force not exceeding 100 oersted.

6. A method of manufacturing a ferromagnetic ferrite having asubstantially square hysteresis loop comprising the steps, forming acompacted mixture of cobalt and iron oxides in proportions at which theatomic ratio CozFe is between about 0.007:1 and 0.4: l, heating saidmixture to a temperature of about l350 to 1450 C. in an atmosphere ofcarbon dioxide, nitrogen and hydrogen to form a coherent body having adensity not less than 90% of theoretical density, and cooling said bodyfrom a temperature exceeding 250 C. in a magnetic field whereby a bodyis formed having an (R max of at least 0.85 and a coercive force notexceeding 100 oersted.

7. A method of manufacturing a ferromagnetic ferrite having asubstantially square hysteresis loop comprising the steps, forming acompacted mixture of cobalt and iron oxides in proportions at which theatomic ratio C0:Fe is between about 0.00721 and 0.421, heating saidmixture to a temperature of about l350 to 1450 C. in an atmosphere ofcarbon dioxide, nitrogen, and hydrogen to form a coherent body having adensity not less :than 90% of theoretical density, heating said body toa temperature of about 600 C. in an atmosphere of carbon dioxide, andcooling said body in a magnetic field whereby a body is formed having an(R of at least 0.85 and a coercive force not exceeding oersted.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Harvey et aL: RCA Review, September 1950, pp. 344- 347.

Philips Technical Review, 1954, pp. 49 to 58.

Physical Review, 1788-1798.

vol. 16, No. 2, August vol. '99, No. 6, Sept. 15, 1955, pp.

1. A FERROMAGNETIC FERRITE BODY HAVING A SUBSTANTIALLY SQUARE HYSTERESISLOOP FORMED BY FIRING A COMPACTED MIXTURE OF COBALT AND IRON OXIDES INPROPORTIONS AT WHICH THE ATOMIC RATIO CO:FE IS BETWEEN ABOUT 0.007:1 AND0.4:1 AT A TEMPERATURE OF ABOUT 1350* TO 1450*C. UNDER REDUCINGCONDITIONS TO FORM A COHERENT BODY HAVING A DENSITY EXCEEDING 90% OFTHEORETICAL DENSITY, AND COOLING SAID BODY FROM A TEMPERATURE EXCEEDING250*C. IN A MAGNETIC FIELD TO THEREBY FROM A BODY HAVING AN (RS)MAX OFAT LEAST 0.85 AND A COERCIVE FORCE OF NOT MORE THAN 100 OERSTED.